The present invention relates to a method of drying lignocellulosic fibrous material in connection with the manufacture of fiberboard, for example MDF (Medium Density Fiberboard). This fibrous material generally consists of mechanically defibered size-coated fibers. For achieving optimum drying results, the drying is carried out in two steps, and with the help of drying gas. In the first step the fibrous material is transported with a first drying gas through a first drying line to a first cyclone, where the drying gas is separated from the material. Thereafter, the material is transported with a second drying gas through a second drying line to a second cyclone, where the drying gas is separated from the material. From the second drying step the material is moved to subsequent steps in the process for the manufacture of fiberboard. During two-step drying in this manner, both steps can be carried out with drying gas, which consists of air, which is heated with flue gas either directly or indirectly, or in some other way.
In dryers of this type great environmental problems arise due to the emission of fiber dust, formaldehyde and volatile organic compounds (VOC). One way to reduce these problems is to recycle the separated second drying gas from the second cyclone to the first step. In order to bring about additional cleaning of the gas emitted from the dryer, however, it is necessary to use separate cleaning equipment, for example gas scrubbers, RTO (Regenerative Thermal Oxidizer) or WESP (Wet Electrostatic Precipitator). This implies, however, that the cost of the drying plant increases considerably.
In accordance with the present invention, these and other problems have been overcome by the invention of a method for drying fibrous lignocellulosic material in connection with the manufacture of fiberboard comprising transporting the fibrous lignocellulosic material by means of a first drying gas to a first cyclone, separating the first drying gas from the fibrous lignocellulosic material in the first cyclone, transporting the fibrous lignocellulosic material separated from the first drying gas by means of second drying gas to a second cyclone, separating the second drying gas from the fibrous lignocellulosic material in the second cyclone, recirculating the separated second drying gas for use as a first portion of the first drying gas, and recirculating the separated first drying gas for use as a second portion of the first drying gas. In a preferred embodiment, the method includes controlling the flow of the recirculating first drying gas so as to avoid condensation in this method.
In accordance with one embodiment of the method of the present invention, fresh air is utilized as a portion of the second drying gas, the fresh air comprising the entire amount of fresh air used in this method.
In accordance with another embodiment of the method of the present invention, the method includes recirculating from 50% to 60% of the separated first drying gas for use as the second portion of the first drying gas.